The present application relates generally to electrophotographic image forming devices, and in particularly to controlling the speed of a media sheet through a fuser so as to control the size of a bubble in the media sheet between a toner transfer station and the fuser
Electrophotographic image forming devices, such as printers, copiers, fax machines, and combinations of these functions (known as all-in-one devices), are a ubiquitous accessory in the office/computing environment. High throughput (i.e., the number of pages printed per minute, or ppm), high image quality, and compact size (i.e., a small desktop “footprint”) are desirable features of electrophotographic image forming devices.
Media sheets, such as paper, transparencies, envelopes, and the like, move along a media path through an electrophotographic image forming device. The media path includes a toner transfer area where toner images are transferred to the media sheets. The toner material comprises fine particles of plastic of particular colors. A media sheet with toner images formed on it then passes through a fuser where heat and pressure are applied to melt the toner and adhere it to the media sheet, permanently fixing the image on the sheet.
A controller may monitor and control the movement of media sheets along the media path. The controller may carefully control the speed of the media sheet at various points to ensure adequate print quality. One area where the speed is carefully controlled is the toner transfer area where the media sheet should move within a specific speed range. If the media sheet moves too quickly or slowly through this area, the toner images are not adequately transferred to the media sheet, which may result in a print defect. The speed of a media sheet through the fuser may also be controlled, to optimally fix the toner to the media sheet.
The media sheet may move at different speeds along different sections of the media path. In particular, the media sheet may move at a different speed through the fuser than it moves through the toner transfer area. In compact electrophotographic image forming devices, the distance along the media path from the toner transfer area to the fuser may be less than the length of a typical media sheet. In this case, a media sheet may be present in both the fuser nip and the toner transfer nip at the same time.
If the fuser nip is driven faster and with a higher nip pressure than the toner transfer nip, the fuser nip may “drag” the media sheet through the toner transfer nip, causing a print defect. If the fuser nip is driven slower than the toner transfer nip, a bend or “bubble” will form in the media sheet between the toner transfer and fuser nips, The size of the bubble depends on the relative speeds of the two nips. Particularly in compact designs, if the bubble is too large, the media sheet may contact elements outside of the media path, which may disturb the toner image on the media sheet, deposit unwanted toner on the media sheet, or otherwise adversely affect print quality. The lower limit on the size of the bubble is zero, at which point the fuser nip may drag the media sheet through the toner transfer nip, causing a print defect. Accordingly, controlling the bubble size is important to maintain image quality, in compact image forming devices where the distance between the toner transfer area and the fuser along the media path is less than the length of a media sheet.